Hi, i wanted to use some Bing API calls (for search and maps) for my WP8 app, but noticed that some people write that you have to pay for using the Bing API, some
others write it is free. What is true?
And do you know a good point (website) to start?

I know that the Maps is free because I used it in my app. But there are usage restrictions- after a certain number of Bing map calls, a user would see a watermark. The number is sufficient for testing and light to moderate use of the app. If suddenly your app becomes extremely popular and thousands of people use it, you will need to pay for the Bing API. More information is at Bing Maps Developer Resources: Map SDKs, APIs, Tips, Training

It is still usable but the entire map control will have a watermark that, I think, says "watermark". I don't know about the Bing search engine. I've never used it but I'm not sure what you want to do with it. Are you trying to create hooks into Bing to search using your own app?